1. Field of Invention
The present invention relates to particle image velocimetry systems and methods of use, and particularly to particle image velocimetry systems that have hollow waveguide illumination systems.
2. Discussion of Related Art
Digital particle image velocimetry (DPIV) is an optical technique for measuring the velocity of particles suspended in a fluid flow field. The measurement area within the flow field is defined by the position and physical dimension of a fan of laser light. For effective digital particle image velocimetry (DPIV) illumination and accurate velocity measurements, the laser illumination system should provide a highly effective delivery of the beam from the laser source to the measurement region as well as the formation of a light sheet that satisfies some specific requirements such as the following: (1) thin (0.5-1.0 mm) but wide (10 mm or wider) laser illumination sheet, (2) uniform laser sheet profile, (3) high-peak-power laser delivery without damaging effects (high-peak-power density greater than 1 GW/cm2), (4) flexible, precise placement, and able to access hard-to-reach regions, and (5) safe and confined laser delivery. According to these requirements, two basic DPIV illumination techniques have been developed recently: a bulk-optics-based illumination technique and a laser delivery technique based on a fiber-optic approach. The bulk-optics DPIV illumination technique utilizes a three-dimensional (3D) articulating arm (TSI, Minneapolis, Minn. and Oxford Lasers, Littleton, Mass.), including tubing, gears, and reflectance optics (a series of mirrors and prisms) to provide for accurate positioning of the high-energy laser sheet. This type of laser delivery technique does not provide a uniform beam because the laser itself is typically multimode and has a very peaked beam profile. In addition, the bulk optics systems are relatively expensive.
These difficulties can be overcome as well as satisfying the above-mentioned requirements when a fiber-optic laser delivery approach is employed. DPIV fiber-optic links have been proposed in recent publications in which the authors have demonstrated the use of either fiber bundles [Anderson D; Jones J; Easson W; Greated C; (1996), “Fiber-optic bundle delivery system for high peak power laser particle image velocimetry illumination,” Rev. Sci. Instrum. 67, 2675-2679; Hunter B; Leong K; Miller C; Golden J; Glesias R; Layerity P (1996), Selecting a High-Power Fiber Optic Beam Delivery System Laser, Institute of America Proceedings 81E, 173-82; Jones J; Anderson D; Greated C; (1997), Fiber-Optic Beam Delivery Systems for Particle Image Velocimetry, Optics and Lasers in Engineering, 27, 657-74; and Hand D; Entwistle D; Maier R; Kujn A; Greated C; Jones J (1999), Fiber Optic Beam Delivery System for High Peak Power Laser PIV Illumination, Meas. Sci. Technology 10, 239-45] or diffraction optics [Stephens T; Haste M; Towers M; Thompson M; Taghizadeh M; Jones J; Hand D; (2003), Fiber-optic delivery of high-peak-power Q-switched laser pulses for in-cylinder flow measurements, Appl. Opt 42, 4307-4314] to deliver high-energy illumination for DPIV systems. The DPIV fiber-optic laser delivery systems offer advantages over conventional bulk-optics-based delivery techniques in terms of effective laser delivery, flexibility, miniaturization, simplified alignment, immunity to external influence (including vibrations and angular laser beam drift), and safe and confined laser delivery. These systems, however, demonstrate some limitations related to possible damage effects of the delivery fiber material when high-peak-power laser emission is used or to beam quality (focus ability) and laser sheet thickness if large-core-diameter fibers (usually exceeding 200 μm) are used. Because these difficulties are caused mainly by limitations of the delivery fiber itself, a detailed evaluation of damage threshold and optimization of critical fiber parameters were studied. Such results have been recently reported in the literature about both commercially available silica solid-core fibers (100-200 μm) and preliminary testing of an experimental prototype hollow waveguide (HW) [Robinson R; Ilev I (2004), Design and optimization of a flexible high-peak-power laser-to fiber coupled illumination system used in digital particle image velocimetry, Rev. Sci. Instrum., 70, 4856-4862]. There is thus a need for improved laser delivery systems for DPIV systems and DPIV systems that have such improved laser delivery systems.